A method of controlling a printer is described, for printing a pattern from a first row of fluid ejection nozzles and a second row of fluid ejection nozzles in a multi-pass printing mode. The first and second rows of fluid ejection nozzles eject fluid of a first type and fluid of a second type, respectively. The method comprises: assigning respective parts of the pattern to be printed to the first and second rows of fluid ejection nozzles; applying a first mask to the first row of said ejection nozzles and a second mask to the second row of fluid ejection nozzles for printing with selected nozzles of each of the rows of fluid ejection nozzles during each pass; wherein the first mask comprises a mesa portion and the second mask comprises a mesa portion wherein the mesa portion of the first mask includes at least a first peak and the mesa portion of the second mask includes at least a second valley, the first peak overlapping with the second valley.